1. Field of the Invention
The present invention generally relates to a drive for coupling torque from one rotatable element to another rotatable element, and more particularly, coupling the torque using a pawl.
2. Description of the Related Art
Starter motors for internal combustion engines are designed to engage a pinion gear, a first rotatable element, with a flywheel gear, a second rotatable element. When engaged and power is applied to the starter motor, the flywheel is turned to start the engine. As soon as the engine starts, the pinion gear must be decoupled from the starter motor; otherwise, the starter motor is driven by the flywheel and may be damaged. The decoupling is conventionally performed using starter motor drive including a sprag or roller clutch. A starter motor having a roller clutch is depicted in FIG. 1. The conventional starter motor comprises a hold-in winding 1, a pull-in winding 2, a return spring 3, an engaging lever 4, a meshing spring 5, a driver 6, a roller-type overrunning clutch 7, a pinion 8, an armature shaft 9, a stop ring 10, a spiral spline 11, a guide ring 12, a terminal 13, a contact 14, a contact break spring 15, a moving contact 16, a solenoid switch 17, a commutator end shield 18, a brush holder 19, a carbon brush 20, a commutator 21, a pole shoe 22, an armature 23, a field frame 24, and an excitation winding 25. The arrangement and assembly of these components is well known in the art. The starter motor drive includes the clutch 7 and pinion 8.
As shown in FIGS. 2 and 3, the sprag and roller clutches 200, 300 respectively, use a wedging action to “lock-up” the clutch. Such action causes the clutch to endure extremely high radial stresses to transmit even a moderate amount of tangential force, or useful torque. Consequently, the components of the clutch (outer race 202; 302; inner race 204, 304; roller 306 and sprag 206) must be fabricated of expensive, high quality bearing steel that is hardened to withstand the forces generated by the wedging action.
FIG. 4 depicts a perspective, sectional view of a portion of a conventional starter drive 400 comprising roller-type overrunning clutch 402 (commonly referred to as a roller clutch) and a pinion 404. FIG. 5 depicts a partial cross-sectional view of the roller clutch 402 and pinion 404. The roller clutch 402 comprises a needle bearing 406 positioned between a 1-way cam 408 and a pinion raceway 410. The clutch 402 further comprises a bushing 412 inside a bore 414 in the pinion 404, a clutch shell 416, a clutch housing 418, a roller retainer 420, a roller spring 422, a mesh spring 424 and a drive flange 426.
As the pinion 404 locks, there is an extreme radial (hoop) stress as the roller 406 is wedged against the cam 408. A special steel, exact machining and expensive heat treatment is used to ensure that the clutch 402 can withstand the stress. Additionally, the structure of the clutch and its assembly procedure is complicated. Furthermore, a clockwise and counterclockwise rotation means a different shell-to-cam orientation is needed for each direction. This requirement adds complexity and manufacturing difficulty. Similar clutches are used in other applications where torque is coupled from one rotatable element to another rotatable element.
Therefore, there is a need in the art for an improved drive for coupling torque between rotatable elements.